Plastic key caps are a great way to make it so you can easily tell your keys apart. They're not expensive by any means, but if you'd like to make your own, Instructables user Andrea Biffi shows how to do it with some Sugru.

All you need to do is cover a key with Sugru and flatten it down. Then, you'll want to add some texture to differentiate the keys. Biffi recommends using something like textured cloth to imprint on the surface, but really anything will work. It's a simple process that'll easily make your keys unique and easy to find. If that's too much work, an old balloon will certainly do the trick.